Band pass amplifier



Dec. 13, 1949 w, w, MOE 2,491,326

BAND PAS S AMPLIFIER Fil'ed May 26. 1945 Inventor: William W. Moe,

b3 Wan 7:5

His, AbtoT-ne g i atented Dec. 13, 1949 Genera New York l Electric Company, a corporation of 4 claims;

Myinvention relates-to interstag'e coupling circuits for amplifiers and it has for one of its ob, .iectstoprovide a certain improvement therein whereby greater transmission gain is had-without complication of the circuits:

mfurtherobjectof theinvention is to providesuch acouplingcircuit having the advantage that its;f-requency of resonance is'less-afiected by variationsin'unidireetional potential applied between thegrid'and cathode of the second stage, as for automatic volume control purposes.

The novelfeatures-which I' believe to be characteristic of 1ny invention= are set forth with particularity in the appended claims. My invention- '1p 1f,- however,= both asto itsorganization and method of operatiom together with further ob j'cts andadvantagesthereof, may best be understood by referencev to the following description taken in connection with the accompanying drawing inwhich--Fig. 1 represents an embodiment of my invention and Fig. 2 represents a d c i -t e t.

Iteferringto Fig; 1,;1- have indicated at I and 2 a; first and second electron discharge amplifier,- the amplifier I having an; anode and cathode be:

tween yvh ich an operating voltage is supplied through a resistance 3:. This source of operatingvoltage may beconnected between the cathodes of the twoamplifiers-, which may be grounded;

andthe conductor 4 and-may be bypassedfor fre- 'quencies at w hich' the amplifieroperates by means-of condenserpfi; The anode of dischargedevice I is-cdupledfor currents of the frequency tobe amplified: to the input of' amplifier 2 by means of the couplingandblocking capacity 6: and inductance coil; I; the grid of the amplifier 2" bing connected tothetap-B on the-coil I:

It has been found that very important advan tages are obtained by connecting thegrid-of the" device 2 to -the tap don the inductance coilrather than to the upper terminal thereof and by the this smaner' capacity being substantially equal to the interelectrode capacity between the anode and all other electrodes of the device I. These two capacities are indicated by dotted lines 9' eezf rei i ex ep eii w i h resonates th the e i ee s, o th eq l -V Thee r sm? course, a large capacity which does not material- 1y influence the frequency of resonance.

If the electron discharge devices employed be of the SAC? typi haying a control electrode and; screen and sup'preassor electrodes; theinterel ec trode capacity between the anode andalr other: elements" of device I may be of the order of five microinicrofara'ds whereas the interelectrode ca-", pacity betweenthe grid and all" other elements ordevice 2'may be-1'5v micromicrofarads. The posi-. tion of the tap 8 should then be such" that" the; impedance of the whole coil I has the same ratiq to-the impedance of the coil b'etween'the tap 8- and; ground thatthei capacity between? the grid and; all other elementsor device 2;.has to the capacity between the anode-,and-all otherielements of device," I; The 15 mf? of-icapacity betweenthe grid and other elements isthen. reflected as 5 ,ugf. acrosstheentire'coi-l I: This 5 itfristheniadded to the 5 ,uuf. of anode capacity of device I to producethe equivalent of l0 fi ofcapacity across the entire coil]; Resistance-v3 may thenbegive'n a value; toadjust the circuit to thei bandwidthwhich it is desired to transmit When the ampli fieris used; asan intermediate frequencyamp'li fier in a television receiver, for example or as anintermediate frequency amplifler of a pulsed carrier wave re r e dw d h Pelee tra smi ted may extend, forexample; from-28 to 32m'eg;acycle Ithas been found that for the same bandwidths and-same Q; ot the tuned circuit comprising capacity 9 and inductance I the'resistance ima-y be substantially double the value which would obtain if the tap twere-moved-tothe upper extremity of the coil 1'. r fl g r The advantage in-employingthe tap 8 can best be understood by considering-the circuit if the tap 8 be movedto the upper extremity of the coil; Let us assumethatsuificient input voltage is then applied to thegrid of tube; I to produce one volt betweenthe anode and cathode of -tube l. Since;v

thecapacitypf condenser 6 is large,-;this one volt;

is then applied directlybetween the grid and cathode of the device 2. The 1'5 at. input capacity between;theg-11id-*-and other elements of the device 2 plus --the 5 #Mf:;,output capacity betweeniano le' a d gthe e e ts qf-.dev. .c 2, then add to 20 d: which resonates; with the inductance: of; the 0011i] at the operating frequency; The resistance 3-isithen given: a value to produce the required-Q01 transmission bandwidth of thecircuit; v

Assuming now that the tap 8 be" adjusted to" a positiQn -suc'hthat the'input capacity ofdevice 2* is reflected across the entire coil 1 with a value equal to the output capacity of device I, or 5 f. Coil 1 may then be adjusted for resonance at the same frequency with this smaller total capacity, 10 rt, which is only half of the capacity with which it previously resonated. Resistance 3 may now be doubled to produce the same transmission bandwidth with the result that the same voltage applied between the grid and cathode of device I produces two volts between the anode and cathode of device I. Now since the ratio of the impedance of the entire coil I to the impedance between the tap 8 and ground is 3 to 1, this voltage is stepped down by a ratio of 1.73 to 1 and the voltage applied between the grid and cathode is the quotient of 2/1.73 or 1.16. Thus a 16% increase in gain per stage is produced by reason of the stepdown position of the tap 8. Thus by using the coil 1 as a stepdown autotransformer of the proper ratio, an increase in voltage amplification is obtained in the amplifier.

In multistage amplifiers such as may be used in pulse receivers this is very important. Thus in an eight-stage amplifier a .gain of 16% in each stage increases the total gain of all of the amplifiers by a ratio of 3.3 to l.

The use of the autotransformer I with the stepdown connection is also of particular importance where variable direct current voltage is applied between the grid and the cathode of the device 2 as for gain control purposes, for example. This connection is shown in Fig. 2 where the lower terminal of the coil 1 is shown connected to ground through the by-pass condenser 10, and to a conductor H on which variable unidirectional voltage with respect to the cathode is applied for gain control purposes. This variable control voltage, of course, varies to some extent the capacity between the grid and cathode of the device 2 but since this capacity as reflected across the entire coil I is reduced to one-third, variations therein are correspondingly reduced and thus effect to a considerably smaller extent the frequency to which the circuit is resonant. With the proportions mentioned, these variations are reduced by about 50%.

While I have shown a particular embodiment of my invention it will, of course, be understood that I do not wish to be limited thereto since different modificationsgnay be made and I contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. An amplifier comprising a first electron discharge device having an anode and cathode, a second discharge device having a grid and cathode, an inductance coil connected between said anode and cathode, a portion of said inductance coil being connected between said grid and cathode, the impedance of said coil having the same ratio to the impedance of said portion that the capacity between said grid and cathode has to the capacity between said anode and cathode.

2. In combination, a first electron discharge device having an anode and a cathode, a second electron discharge device having a grid and a cathode, a coupling between the anode and cathode of said first device and the grid and cathode of said second device comprisinganinductance coil connected between said anode and cathode, a portion of said inductance coil being connected between said grid and cathode, the impedance of said coil having the same ratio to the impedance of said portion that the capacity between said grid and cathode has to the capacity between said anode and cathode, and said inductance coil being resonant with said capacities at the operating frequency of said amplifier, said capacities and the distributed capacities of said coil and circuits connected thereto comprising the sole capacity resonating said coil, and a resistance across said coil proportioned in accord with the frequency bandwidth transmitted by said coupling.

3. In an amplifier comprising a first stage having an anode and cathode and a second stage having a grid and cathode, said cathodes being connected together, and an inductance coil connected between said anode and cathode the voltage on which is supplied between said grid and cathode, the combination of means to increase said voltage comprising a tap from said grid to a point on said coil so positioned that the impedance of said coil to the impedance between said tap and said cathodes has the same relation that the capacity between said grid and cathode has to the capacity between said anode and cathode, said coil being resonant with said capacities at the frequency which said amplifier operates, and a resistance connected across said coil having a'value to produce desired bandwidth of oscillations transmitted between said amplifier and greater than the value required to produce the same bandwidth were the whole of said coil connected between said grid and cathode.

4. The combination, in an amplifier comprising a first stage having an anode and cathode and a second stage having a grid and cathode, the capacity between said grid and cathode being greater than that between said anode and cathode, a source of operating potential connected between said anode and cathode through a resistance, a capacitance and an inductance coil connected in'series between said anode and cathode, said grid being connected to an intermediate point on said inductance so positioned that the inductance of said coil has the same ratio to the I inductance between said intermediate point and and means to supply a variable unidirectional voltage between said grid and cathode to vary the amplification of said second stage whereby the variation in frequency to which said coil is resonate produced by variation in said unidirectional voltage is reduced dueto reflection by said The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,959,062 P1308 May 15, 1934: 2,226,739 Percival Dec. 31, 1940 

